I have spent 40 years pulling up nettles in my garden and cutting them in the pasture (the goats will eat them once they have wilted).
It was only this week that I finally looked at them properly. It turns out that some nettle plants are male, and some are female. The flower stalks on the male plants are more or less horizontal, and if you look very carefully, you can see tiny, tiny little white flowers, half the size of a pin-head, on them.. These flowers are obviously much too small for bees to polinate - but some of the nettle flowers are covered in little black beetles. Looking more closely still, I found lots and lots of ladybirds, green aphids, grey aphids, white aphids, and lots and lots of other insects on the leaves and stalks. Even the occasional little snail and one chesnut-brown slug!
The flower stalks of the female nettles are very drab, and hang down like skirts. There are minute specks of white on them, which are probably the flowers, but I can't see them clearly.
Later in the year, when the peacock butterflies are out, look again at the nettles: the butterflies lay their eggs on the nettle plants, and the caterpillars eat the leaves.